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Chapter 7: Periodic Properties
“Well, Pod 6 is just one big party, huh? Big fat fun.”
The Shielding Effect and Effective Nuclear Charge Electrons are negative; the protons in the nucleus are positive. The electrons repeal each other, and in doing so screen or shield each other from the full positive charge of the nucleus. Electrons on the inside screen outside electrons.
Beryllium (1s^2 2s^2 ) has four protons and two inner electrons. The inner electrons screen two protons, leaving the outer electrons seeing two protons (the outer electrons don’t shield each other).
Nitrogen (1s^2 2s^2 2p^3 ) has seven protons and two inner electrons. The inner electrons screen two protons, leaving the outer electrons seeing five protons (the outer electrons don’t shield each other). Nitrogen’s electrons are feeling a greater ENC than are beryllium’s.
ENC=(# of protons in the nucleus) – (# of shielding electrons: how many electrons does the noble gas core have?)
ENC tends to increase moving from left to right across a row – shielding is staying more or less constant (“push”) while the number of protons is increasing (“pull”).
Atomic Radius The size of an atom.
- Left to right – ENC on the outer electrons is increasing; the electrons don’t roam as far; size decreases.
- Top to bottom – ENC constant, but size of electron cloud is increasing (n is increasing); size increases.
Ionic Radius The size of ions. Follows the same basic rules as atomic radius, with some other things to remember.
- Positive ions – losing electrons lowers the amount of electron-electron repulsion going on; cloud shrinks; positive ions are smaller than the parent ground-state atom.
- Negative ions – added electrons increase the amount of electron-electron repulsion going on; cloud gets bigger; negative ions are larger than the parent ground-state atom.
Ionization Energy The energy required to strip an electron from an atom. Always positive; electrons don’t just fall out; they have to be knocked out. An atom that has multiple electrons has multiple ionization energies; the first ionization energy is the energy to remove the first electron, the second ionization energy is the energy to remove the next electron…
- Left to right – ENC is increasing; the nucleus has a tighter grip on its electrons; ionization energy increases (w/ some exceptions).
- Top to bottom – Size of electron cloud increasing; electrons that are more distant from the nucleus aren’t as tightly held by the protons; ionization energy decreases.
Electron Affinity Actually measuring sort of the same thing as ionization energy, except backwards. An electron is added to an atom – some atoms release energy (negative EAs), some atoms absorb energy (positive EAs). Negative EAs indicate the atom “likes” extra electrons; Positive EAs indicate the reverse.
- Left to right – EA increases (that is, it becomes a larger negative number, w/ exceptions).
- Top to bottom – EA sort of wobbles all over the place.
Metal vs. Nonmetal Sort of a composite property. Metals have lower EAs and ionization energies. Nonmetals have high EAs and high ionization energies. The most metallic elements are in the bottom left of the periodic table. The most nonmetallic elements are in the top right of the periodic table.
